1. Field of the Invention
The present invention refers to a new curing composition for phenolic resins, especially for the polymerization of resoles and the formation of novolaks.
2. Description of the Prior Art
Phenolic resins have become known for a rather long time. They may generally be classified in two groups, namely the resoles and the novolaks. The resoles are obtained by a condensation reaction starting from optionally substituted phenols and aldehydes, in particular formaldehyde, in a basic medium, whereas the condensation of the same starting products but in an acidic medium and generally in using other stoichiometric ratios leads to novolaks which may be hardened by crosslinking at elevated temperatures in the presence of formaldehyde or a formaldehyde liberating compound, for example hexamethylene tetramine.
For their use in practice, the phenolic resins are crosslinked or cured to the so-called phenoplasts which may be hard foams, composite products that contain extenders or fillers such as bakelite, or others. Certain phenolic resins are self-curing, especially such resins that are crosslinking via their reactive methylol groups, but this self-curing does not yield well-defined end products, and the hardening process is very slow.
The crosslinking of resoles and the formation of novolaks occur according to acidic mechanisms. To this end, sulfuric acid or toluene sulfonic acid are used for example. However, the use of these acids has the major disadvantage that these acids are imprisoned as such within the forming polymers during the polycondensation and crosslinking reactions; the finished products will therefore contain free acids that are very difficult to remove from the product and cause slow decomposition reactions. Therefore, other possibilities have already been proposed in order to overcome this drawback.
U.S. Pat. No. 3,870,661 (to P. J. Crook and S. P. Riley) discloses a foamed phenol-formaldehyde resin, obtained by reacting a resole resin with a sulfonated novolak in the presence of a surfactant and a foaming agent. Furthermore, U.S. Pat. No. 3,872,033 (to P. Boden, P. J. Crook, M. E. Hall and S. P. Riley) discloses a solid fire-retarding phenol-formaldehyde resin, obtained by reacting a resole with a hardening composition produced by reacting a sulfonated phenol with formaldehyde, in the presence of a surfactant and a foaming agent.
The hardening compositions or catalysts, respectively, used in these two documents are composed, as judged by the present inventors, of sulfonated linear phenolic oligomers. In fact, the above two patents do not disclose or suggest any concrete composition or configuration of these hardeners. The one skilled in the art knows that under the reaction conditions described in the two patent specifications, linear phenolic oligomers are formed that are sulfonated after or before the oligomerization reaction using phenol and formaldehyde.
These reaction conditions further comprise reaction temperatures and reaction times during the condensation of phenol and formaldehyde of at most 130xc2x0 C. and 65 minutes in Crook et al. and 50xc2x0 C. and 2 hours in Boden et al. Where para substituted phenols are used in Crook et al., which would be a necessary condition for producing cyclic phenolic oligomers, the reaction conditions are 75xc2x0 C./45 minutes, absolutely insufficient for the formation even of traces of cyclics.
The hardening compositions and curing catalysts, respectively, that are known from the above discussed prior art suffer from the disadvantage that their composition will change within wide limits when the reaction conditions, such as temperature, time and the concentrations and relative amounts of reactants, are not strictly the same from one batch to another. Since the chemical and most physical properties of the compositions are not disclosed (with the exception of solids content of aqueous solutions and their viscosity) and thus cannot be reproduced, the properties of phenolic foams produced from these compositions will widely vary too. This fact follows directly from the lecture of both U.S. Patents referenced above.
The present invention aims at eliminating the disadvantages of the known bridging compositions for resoles or condensation catalysts for novolaks by replacing the acids or the sulfonated linear phenolic oligomers that have been used until now, to improve the polycondensation and crosslinking processes, and to further improve the quality and usefulness of the finally obtained phenolic resins and novolaks. This object is attained by a new composition which contains (A) a first component selected from partially sulfonated cyclic phenolic oligomers and totally sulfonated cyclic phenolic oligomers, and (B) a second component selected from partially sulfonated linear phenolic oligomers and totally sulfonated linear phenolic oligomers, the weight ratio of component (A) to component (B) being comprised between 1 to 99% of component (A) and 99 to 1% of component (B).
The present invention further provides a curing composition, useful as a bridging agent in the polycondensation of resoles and in the formation of novolaks to be produced from a phenol and an aldehyde, said curing composition comprising an intimate blend of (A) from about 1% to about 99% of a first component selected from partially sulfonated cyclic phenolic oligomers and totally sulfonated cyclic phenolic oligomers, produced by reacting a para substituted phenol with formaldehyde in the presence of a basic catalyst and under substantially anhydrous conditions at temperatures of at least about 135xc2x0 C., recovering cyclic phenolic oligomers from the reaction mixture, and partially or totally sulfonating these cyclic phenolic oligomers, and (B) from about 99% to about 1% of a second component selected from partially ant totally sulfonated linear phenolic oligomers produced by preparing linear phenolic oligomers from a phenol and formaldehyde in the presence of a basic or an acidic catalyst, recovering linear phenolic oligomers from the reaction mixture, and partially or totally sulfonating these linear phenolic oligomers.
Furthermore, this invention also includes a phenolic resin obtained by a polycondensation of a resole, the phenolic resin comprising, chemically incorporated into the microstructure of the polymer, a curing composition consisting of from about 1 to 100 % of at least one partially or totally sulfonated cyclic phenolic oligomer; that composition may further contain, if the percentage of said partially or totally sulfonated cyclic phenolic oligomer is lower than 100%, at least one partially or totally sulfonated linear phenolic oligomer and/or sulfonated monomeric phenol.
In an analogous manner, this invention also includes a novolak obtained by a polycondensation of an aldehyde, a phenol and a catalyst composition, the novolak comprising, chemically incorporated into its microstructure, said catalyst composition which consists of from about 1 to 100 % of at least one partially or totally sulfonated cyclic phenolic oligomer; that composition may further contain, if the percentage of said partially or totally sulfonated cyclic phenolic oligomer is lower than 100%, at least one partially or totally sulfonated linear phenolic oligomer and/or sulfonated monomeric phenol.
In this document, all percentages refer to the weight if not otherwise specified.
In a general manner, the sulfonated cyclic phenolic oligomers are called xe2x80x9ccalixarene sulfonic acidsxe2x80x9d and are produced by a dealkylating sulfonation of p-alkyl substituted calixarenes, or by first dealkylating such compounds and the sulfonating them. Details are discussed below.
Although it may be imagined that cyclic phenolic oligomers may theoretically be formed in the processes of the above discussed Crook and Boden references, the one skilled in the art knows that the amounts thereof possibly formed during the reactions which are disclosed, would be undetectably low since the reaction conditions, even if they were drastically reinforced, do not allow the cyclization of para unsubstituted phenolic oligomers; as to para substituted ones, anhydrous conditions are required, not contemplated in the references which do moreover not disclose or suggest any cyclic compound. Furthermore, the invention does not speculate upon the possible presence of imaginary compounds in the bridging compositions but contemplates the addition of determined amounts of calixarene sulfonic acids, having a determined structure and composition, to the bridging compositions; otherwise, the purpose of the invention, namely the provision of well determined compositions for making well tailored phenolic resins and novolaks in a fully reproducible manner, could not be attained.
The condensation of the phenol and the formaldehyde (formol) may be conducted under predetermined stoichiometric conditions in the presence of an acid or a base in such a manner that more or less condensed linear phenolic oligomers are obtained. The sulfonation of these oligomers, as such or as a mixture, yields partially or totally sulfonated linear phenolic oligomers, see the Crook and Boden references discussed above.
The sulfonated cyclic phenolic oligomers must be prepared from linear phenolic oligomers wherein the starting phenol carries a para substituent, preferably a tert-butyl group which is then removed prior to or during sulfonation and is partially or totally replaced by the sulfonic acid group SO3H. These substances are already partly known, see, for example, the paper xe2x80x9cSynthesis and acid-base properties of calix [4], calix [6] and calix [8] arene p-sulfonic acidsxe2x80x9d by J. P. Scharff, M. Mahjoubi and R. Perrin, New J. Chem. 1991, 15, 883-887. The cyclic phenolic oligomers are called xe2x80x9ccalixarenesxe2x80x9d, and the number of phenolic nuclei per molecule is inserted between brackets in the name. The synthesis of calixarenes calls for relatively severe reaction conditions, namely at least 135xc2x0 C./2 hours in an anhydrous environment from which reaction water is continuously to be removed.
In a surprising manner, it has now been found that these sulfonated phenolic oligomers that are introduced into the resoles and/or used for the formation of novolaks instead of the acids (or the exclusively linear sulfonated phenolic oligomers) used until now, allow to obtain phenolic products having new, advantageous and unexpected properties. During the polycondensation of resoles to crosslinked or cured products, these substances replace at least partially but preferably totally the conventional catalyst acid; they do not remain as such within the crosslinked network but are chemically combined in the molecular network structure. When used for the formation of novolaks, they replace in part or better completely the usual catalyst acids and are also incorporated into the molecular chemical structure of the novolak.
In the present document, the composition of matter which is dealt with is named xe2x80x9ccuring compositionxe2x80x9d and is to be understood for comprising the crosslinking agents in the case of resoles and the xe2x80x9ccatalystxe2x80x9d of the novolak formation as well. This composition is defined by at least one partially or totally sulfonated cyclic phenolic oligomer, optionally also containing at least one partially or totally sulfonated linear phenolic oligomer, and/or a sulfonated phenol. By xe2x80x9cphenolicxe2x80x9d and xe2x80x9cphenolxe2x80x9d, respectively, a compound is to be understood which has at least one aromatically unsaturated nucleus, substituted by at least one OH group and having unsubstituted ortho positions; this definition also includes phenols that are substituted in meta and/or para positions by classic radicals such as alkyl, alkenyl, aryl, ether, nitro, halogen, etc.; these substituents may be selected at will under the only condition that they must not impair the formation of the desired resoles and novolaks. These substituents may be used for conferring special properties to the final product; the one skilled in the art knows the principles of the action of particular substituents.
In the composition of this invention, the linear and cyclic oligomers may be totally or partially sulfonated. The ratio between sulfonated linear oligomers, sulfonated phenol and sulfonated cyclic oligomers may vary between all values from about 1 to about 99 % by weight. In the compositions, the mixture of linear and cyclic sulfonated phenolic oligomers, containing linear and cyclic entities in any proportion between 1 and 99%, preferably between 5 and 95%, is generally obtained by blending linear and cyclic oligomers, separately prepared and separately sulfonated; this way of action is preferred when the use of the mixture must closely be tailored to certain requirements of use. On the other hand, it is also possible to prepare a mixture of linear and cyclic phenolic oligomers, synthesized separately, and to sulfonate this mixture, the para dealkylation of the cyclic phenolic oligomers being carried out before or after preparing their blend with the linear entities.
Further preferred compositions contain from about 20 to about 50% of sulfonated monomeric phenol, the remainder being substantially equal parts of partially or totally sulfonated linear phenolic oligomers and of partially or totally sulfonated cyclic phenolic oligomers.
An important advantage of the compositions according to the present invention is that they are normally water soluble. Depending on the proportion of an aqueous solution of sulfonated linear oligomers and/or phenol, added to the cyclic oligomers, the viscosity of the crosslinking agent in the case of resoles can be modified and adjusted. The final viscosity of the crosslinking agent, constituted by the above mentioned mixture of sulfonated phenolic oligomers, may generally vary between 0.03 Paxc2x7s and 3 Paxc2x7s, measured at 20xc2x0 C.
The use of the totally or partially sulfonated cyclic phenolic oligomers (calixarenes) as a crosslinking agent or a condensation catalyst according to this invention allows firstly to accelerate the reaction speed; mixtures with sulfonated linear phenolic oligomers in ratios of 0.5 to 5% will reduce the crosslinking time of resoles until a fifth of the duration required until now. Used as such or in mixtures, the sulfonated calixarenes permit the condensation of phenol and formol under the formation of novolaks; compared with the current use of p-toluene sulfonic acid, the condensation time is divided by about 4.
Secondly, the use of totally or partially sulfonated cyclic phenolic oligomers (calixarenes) according to the invention allows to better control the crosslinking and condensation processes. The crosslinking of the resoles and the formation of novolaks occur according to acid type mechanisms. The sulfonated cyclic phenolic oligomers show changeable acid-base properties due to their varying acidities. For example, the calix [4] arene-p-sulfonic acid has four strong acidities due to the SO3H groups and one strong acidity of the OH group (superacid), and three very weak acidities of a pK greater than 11.
The use of totally or partially sulfonated cyclic phenolic oligomers (calixarenes) according to the invention further allows to reinforce the properties of the phenolic resins. The sulfonated phenolic oligomers, having the same nature as the phenolic resins, namely the resoles and the novolaks, are chemically incorporated into the microstructure of the polymer. They are perfectly compatible with the resins and enhance their mechanical and fire retarding properties. They are thermally and chemically stable and have high melting points. For example, the thermographimetric analysis of hydrated calix [6] arene p-sulfonic acid shows the high stability of the product. A first liberation of water occurs at about 90xc2x0 C. only, then a second liberation of water at about 250xc2x0 C. constituting a level that corresponds to the loss of 7 water molecules which is the number of water molecules associated to the calixarene, thus explaining the fire retarding and fire proof properties. The melting point is above 300xc2x0 C., and the product does not decompose until 800xc2x0 C.
Finally, it has been found that the use of totally or partially sulfonated cyclic phenolic oligomers (calixarenes) according to the invention allows to obtain phenolic materials having selective complexation properties; the sulfonated cyclic phenolic oligomers which are introduced into the condensation and crosslinking steps of phenolic polymers bring about, further to their acid functions, potential complexation sites. Due to their cavities or fractions, the calixarenes are privileged and selective sites for the complexation of ions, metals or weakly polar or even neutral molecules. For example, cone configured calix [4] arenes complex the cations Li+, Na+, K+, Rb+, Cs+; Ti 4+, Fe 3+, Co 2+; certain neutral molecules such as toluene, xylene (selectively p-xylene); calix [6] arene p-sulfonic acid complexes UO2+; and calix [8] arenes allow to complex Eu3+and other lanthanides. These selectively complexing properties of cyclic phenolic oligomers yield for example phenolic materials having recognizing, separating, depolluting, confining and other capacities.